The present invention relates to novel composite polyester films having particular surface properties and to their use as protective layers for photopolymer plates.
It is known that photosensitive elements, such as the photopolymer plates used for preparing printing plates and in particular flexographic printing plates, consist of at least one assembly comprising a layer of a photopolymerizable polymer deposited between a carrier and a protective film. In general, the carrier and the protective film consist of a metal film or of films or sheets of film-forming thermoplastic polymers. Polyester films, and in particular orientated polyester films containing several units derived from an alkylene terephthalate, are a preferred material for producing said carriers or protective films for photosensitive elements. The main role of the protective film is to protect the photopolymer layer from the damage which it might sustain during the handling and/or storage of the photopolymerizable elements; it also acts as a separator when the photopolymerizable plates are stacked. The protective film is separated from the photopolymerizable layer at the moment when the photopolymerizable elements are used. In this use, it has been found advantageous to give the photopolymerizable layer a surface topography such that the plating of the surface of the element on the glass plate of the printing machines and/or on the surface of the negatives is as complete as possible.
Such an objective can only be achieved rapidly if the air trapped between the photopolymerizable layer and the glass plate can easily escape when the polymerizable element is placed in position, i.e. if the photopolymerizable layer is sufficiently rough. The protective film can advantageously impart this roughness to said layer by the transfer effect if the surface of said film in contact with the photopolymerizable layer comprises several protuberances having substantially the same height and distributed uniformly over the surface of the protective film in contact with the photopolymerizable layer. Moreover, for certain applications (for example the manufacture of offset plates) for which the photopolymerizable layer is particularly thin (its thickness can be about 3 .mu.m), care should be taken to ensure that the rough surface of the protective film is devoid of peaks which would perforate the photopolymerizable layer. The production of polyester films having such surface properties involves the use of fillers which satisfy particular requirements in respect of their particle size, the particle size distribution and their ability not to reagglomerate when they are introduced into the polyester. Such fillers are generally introduced throughout the thickness of the protective film (so-called full-bulk film), in which case part of the fillers consumed, which does not participate in achieving the desired result, is totally wasted. Furthermore, the use of a high concentration of filler throughout the thickness of the film helps to make said film opaque, which does not allow the photopolymerizable layer to be seen through the protective layer. Finally, the scraps of full-bulk films containing a high concentration of fillers are difficult if not impossible to recycle.